Extrude/Scale evenly in a mirrored circle
Pivot in the circle's center

The origin of the trasformation in your current setup is probably set to "median point", so the scaling is using the baricenter of the geometry as pivot point.

This is usually not a problem while scaling a full circle, as the baricenter of the whole points is in the exact location of the center of the circle.

If you are controlling just half of the point, you should switch to another type of pivot point, that allows you to set the center of the circle as pivot for the transformation.

I'll suggest to use the 3D cursor as shown in the image sequence below: